In recent years, it has been a common practice to install 275 to 500 kV level power cables with the growing power demand. Examples of the power cables which have been put into practical use include conventional kraft insulating paper OF or POF cables, so-called semisynthetic paper (laminated paper)-insulated extrahigh voltage OF or POF cables such as silicon-grafted polyethylene laminated paper (SIOLAP)-insulated OF cable, polypropylene-laminated paper (PPLP)-insulated OF cable, PPLP-insulated POF cable, biaxially-oriented polypropylene-laminated paper (OPPL)-insulated OF cable, OPPL-insulated POF cable and ethylene tetrafluoride-polypropylene hexafluoride-laminated paper (FEP)-insulated OF cable, and crosslinked polyethylene-insulated CV cables. Particularly, it is confirmed that polypropylene-laminated paper-insulated OF cable can be put into practical use as 800 kV OF cable.
Furthermore, insulating materials for solid or mass-impregnated cable are now under extensive examination.
The future tendency in requirements for power cable is for higher transmission capacity, higher applied voltage and longer distance of power transmission. In order to meet this demand, it is necessary that the proportion of the plastic film layer in a sheet of a semisynthetic paper be raised so that the barrier properties against electrical stress of the semisynthetic paper can be enhanced, compensating the difficulty in the enhancement of dielectric strength due to the porosity of the paper and hence providing a high dielectric strength. To this end, it is necessary to provide a sandwiched structure obtained by gluing a kraft paper to both surfaces of a plastic sheet or a one-sided structure obtained by laminating a sheet of a plastic sheet and a sheet of a kraft paper in order to reduce the total thickness of a sheet of the semisynthetic paper, whereby the thickness of the insulating layer in the cable is reduced, thereby providing a compact cable, and thereby the length of the cable having the laminated paper wound therein is increased. One of great difficulties in the preparation of these laminated papers is how to physically glue the kraft insulating paper to the polymer layer with sufficient adhesive strength.
Since the cellulosic fiber constituting the kraft insulating paper has no heat-fusibility, it cannot be molten or chemically bonded or glued to the polyolefin resin film layer at the temperature where the polyolefin resin to be laminated therewith is melt-extruded into film. In other words, the general mechanism of bonding of the cellulose fiber constituting the kraft paper to the melt-extruded film of polyolefin resin is a so-called anchoring effect involving the entry of a high temperature molten polyolefin resin into fine porous spaces produced by the entanglement of cellulose fibers on the surface of the kraft insulating paper.
However, the conventional process for the preparation of a laminated paper which comprises simply melt-extruding a polyolefin resin onto a kraft insulating paper to effect adhesion by means of heat melting of such polyolefin resin is disadvantageous in that the kraft paper is easily peeled off the polyolefin resin film at a step of applying the laminated paper thus prepared to a power cable as an insulating layer and the laminated paper thus obtained is also liable to peeling even after wound on a conductor and impregnated with an insulating oil. The resulting cable has deteriorated properties and thus lacks reliability from the standpoint of long-term stability of insulation.
In order to prevent the insulating paper from being peeled off the polyolefin resin film, it may be proposed to use a technique of coating the surface of the kraft insulating paper with an anchor coat agent such as isocyanate or a corona treatment technique, which has been put into practical use in the art of packaging material. However, such an anchor coat agent is a polar material and therefore has a disadvantage in that it deteriorates the dielectric properties of the electrical insulating laminated paper. Further, the corona treatment technique is disadvantageous in that it makes pinholes in the kraft insulating paper or causes the generation of functional groups (polar groups) such as carbonyl group, carboxyl group and amino group on the surface of the kraft insulating paper which then deteriorate the dielectric properties of the electrical insulating laminated paper. Thus, the corona treatment technique is unsuitable for insulating materials for high voltage apparatus requiring a low dielectric dissipation factor.
As an approach for enhancing the dielectric strength by raising the proportion of the plastic film layer in a sheet of a semisynthetic paper, the reduction of the thickness of the kraft insulating paper forming the laminated paper has been proposed (see JP-B-61-45328 (The term "JP-B" as used herein means an "examined Japanese patent publication")). In general, an easy method for providing a thin laminated paper is to select a thin kraft paper.
A capacitor paper belongs to the group of thin kraft papers. It is said that the lower limit of the thickness of the capacitor paper is from 6 to 7 .mu.m. In general, a thin capacitor paper is prepared by a process which comprises raising the beating degree of a pulp, making a base paper from the pulp, and then subjecting the base paper to secondary processing, i.e., calendering or supercalendering which is even more effective for provision of smoothness. The product thus obtained is a paper having apparently small unevenness and high smoothness. From the standpoint of properties, this paper has a high density and a high air permeability.
As previously mentioned, the mechanism of bonding of the kraft insulating paper to the molten polyolefin film layer is an anchoring effect alone. However, in the production of the thin capacitor paper, calendering or supercalendering is indispensable as described above, and the thus-prepared thin capacitor paper does not have surface unevenness sufficiently. Therefore, when the molten polyolefin resin is laminated with the thin capacitor paper, anchoring effect cannot be exerted since there is an extremely small amount of porous pits into which the molten resin can enter. As a result, only a laminated paper having a low adhesive strength can be obtained. In other words, the prior art techniques have a disadvantage in that the use of a thin kraft insulating paper gives an insufficient adhesive strength with the plastic film layer to be laminated therewith.